What is long term fuel trim?
This article deals with long term fuel trim. If you’d like to learn more about fuel trim in general, read this post
Powertrain and engine control computers (PCM/ECM) employ a long term fuel trim delivery strategy that learns to self-adjust as driving conditions change and the engine wears. Once the engine is up to operating temperature, the PCM/ECM employ a short term fuel trim (STFT) strategy. But as the PCM/ECM begins to notice a pattern of adjustments, it stores those adjustments in a cell similar to an Excel spreadsheet.
These adjustments are referred to as long term fuel trim.
Unlike STFT that changes rapidly as the driver changes accelerator pedal position or the vehicle encounters a rising grade and needs more fuel to maintain speed, LTFT remains a bit more stable for each cell condition.
How long term fuel trim works
Let’s say that the PCM/ECM has found that over time, a worn engine needs 5% more fuel when the engine is running at 1,000RPM and the driver is requesting 30% throttle. Once the PCM/ECM detects those conditions, it refers to its internal lookup table to see if it’s stored a LTFT for that condition. If so, it automatically provides 5% more fuel (or whatever LTFT adjustment is stored).
When reading a scan tool, LTFT won’t change as rapidly as STFT at or near idle. However, if you drive with the scan tool connected, the LTFT will change as the PCM/ECM employs the LTFT adjustments for each cell.
Long term fuel trim is stored in adaptive memory
For more information on how adaptive memory works, read this post
How much LTFT is too much?
Each car maker is different, but the general rule is that a LTFT of +25% or -25% indicates a serious problem. Let’s take a look at the possible causes of a high LTFT.
What causes a high long term fuel trim?
In this example we’ll examine the possible causes of a +13% long term fuel trim. What the LTFT is telling us is that it has to continuously add 13% more fuel. In other words, the engine is running too lean and the PCM/ECM is forced to compensate by adding more fuel.
• Restricted fuel system. Here, the PCM/ECM has commanded a certain amount of fuel based on an expected fuel pressure, But the PCM/ECM isn’t seeing the expected performance and emissions result, so it is forced to extend fuel injector open time to compensate for the lower fuel delivery.
Low fuel pressure—conduct a fuel pressure test
Clogged fuel filter—clogged filter reduces fuel volume but not pressure. Change fuel filter
For more information on how fuel pumps work and failure symptoms, read this post
For information on how to test a fuel pump, read this post
• Vacuum leak. Here the PCM/ECM is basing fuel delivery on feedback from the engine’s mass airflow (MAF) or manifold absolute pressure (MAP) sensor. Any air leak that occurs AFTER these sensors is a leak that’s allowing UNMETERED air into the engine. In other words, the PCM/ECM is expecting a certain volume of air and meters fuel accordingly. But the leak is allowing more air into the engine and the PCM/ECM is seeing that it’s not providing enough fuel. So it adds 13% more fuel to compensate.
Vacuum leak in a vacuum hose, intake gasket or air duct after the MAF sensor. Check using propane enrichment or leak test with a smoke machine.
• Clogged fuel injectors. Here, the PCM/ECM is commanding a certain fuel injector open time which it expects to deliver a certain amount of fuel based on expected fuel pressure. However, if carbon deposits are restricting the flow of fuel through the injector, it delivers less than the expected amount of fuel so the PCM/ECM must add 13% more fuel injector open time to compensate.
Clogged fuel injector port. Test by attaching a fuel pressure gauge. Prime the fuel rail by turning the key to the ON position for 2-secs. Then activate each fuel injector and look for a consistent pressure drop. The fuel injector that causes a lower pressure drop is the one that’s clogged. Remedy? Try adding a bottle of fuel injector cleaner to your gas tank. If that doesn’t solve the problem, have a shop perform a full injector cleaning service.
For information on fuel injector cleaning, read this post
• MAF sensor issue. Here the PCM/ECM is basing fuel delivery on feedback from the MAF sensor and assumes the reading is accurate. But the hot wire system used in MAF sensors can accumulated baked on deposits that affect its readings. A dirty MAF sensor typically OVER ESTIMATES the amount of air entering the engine at idle and UNDER ESTIMATES the amount of air entering the engine at cruise speeds. In other words, the PCM/ECM expects a certain air volume but is getting more air or less than it expected so it adds 13% more fuel.
Dirty MAF sensor. Try cleaning. Defective MAF. Replace
For more information on how to clean a MAF sensor, read this post
• Exhaust leak. Here the PCM/ECM is relying on accurate O2 sensor readings to determine how well it did in estimating fuel delivery. An exhaust leak can cause false O2 sensor readings because it allows excessive amounts of oxygen into the exhaust. As the PCM/ECM sees excess oxygen, it compensates by adding 13% more fuel.
Leak in the exhaust at the cylinder head, flexible joints, holes in the pipes before the upstream O2 sensor. Smoke test the exhaust system to find the leak.
• Engine misfire. Here the PCM/ECM expects a certain result but finds excessive oxygen in the exhaust stream due to a cylinder misfire. A misfire is incomplete combustion, so it will force too much air into the exhaust that is picked up by the O2 sensor. The misfire can be caused by a faulty ignition system or a mechanical problem such as a chipped valve, broken valve spring, faulty valve seat, lifter or camshaft issue.
Worn or faulty spark plug or spark plug wire. Faulty ignition coil. Mechanical problem with the cylinder head, valves, seats, springs, lifter or camshaft.
For tips on how to diagnose a misfire, read this post
• Secondary air failure. Here, the PCM/ECM is expecting the secondary air pump to blow outside air into the catalytic converter at cold startup open loop to provide extra oxygen. The extra oxygen is needed to catalyze the extra fuel needed at cold start. The PCM/ECM monitors both the upstream and downstream O2 sensors and expects certain readings. If the secondary air system is inoperative, the downstream O2 sensor will read rich. If the system is leaking, it will allow excess air into the catalytic converter at all times, causing a lean report from the downstream o2 sensors.\
©, 2018 Rick MuscoplatPosted on by Rick Muscoplat